Telescopes between 20 and 100 meters in diameter are now being designed by the astronomical community. The telescopes are asked to achieve near-diffraction-limited performance. It is a tremendous challenge, since the larger the telescope, the harder diffraction-limited-performance is to achieve. New technology is needed. By using “transfer mirrors” similar to those described in this application this objective may be achievable. These new mirrors can be light weight, can be designed for use as either active or active/adaptive optics, and have important figure control advantages over mirrors in present large telescopes. Active optics involves the control of the optical figure by using actuators to push on the back of the mirror. These actuators are typically very powerful and slow. Adaptive optics are used to also correct the effect of the atmosphere on images formed by light that has passed through a significant thickness of air. Ideally their response time should be shorter than a millisecond. Previous actuators have typically been large, slow (compared to a millisecond) expensive devices. Usually they have been stacks of piezoelectric plates and the movement occurs because of the expansion or contraction of the piezoelectric material itself. Very large voltages are required to obtain displacements of several micrometers. The actuators in the mirrors are discs. The bowing of the disc magnifies the displacement of an actuator rod placed at the center of the disc. Voltages required are low, in our case ±50V. Also, since they are very light, the frequency response of these actuators is typically considerably shorter than one millisecond.
The scattered light level of these transfer mirrors will be over 10× less than normal telescope mirrors, a factor that is important in resolving dim objects near a bright object. The composition mirrors discussed here will have a low expansion coefficient similar to that of glass ceramics with extremely low thermal expansion coefficients such as Zerodur™ or ULE™, the lowest expansion coefficient materials known. Low expansion is important to obtain stable telescope operation at different temperatures. Since the composite mirrors can be relatively lightweight, and can also be adaptive with similar time constants, they become an enabling technology for constructing very large telescopes.
One application of these mirrors will be as beam directors used to transmit laser energy from the ground to power orbital transfer vehicles (OTV) in space. These OTV's can carry satellites from low earth orbit into mid-earth or geosynchronous orbit at a fraction of the cost of present chemical rockets, assuming that they are powered by ion engines. However these ion engines require large amounts of power. For example, a magnetoplasmadynamic (MPD) ion engine developed at the Jet Propulsion Laboratory in Pasadena, Calif., developed a thrust of 12.5N but required a power of 200 kW. Typical satellites in space generate powers of approximately 5 kW and it has been believed that nuclear electric generators in space will be required to use the MPD or the competing Hall Thruster technology. That conclusion is challenged by the development of large mirrors built up from the lightweight adaptive optic segments described in this application and powerful free electron lasers (FEL). An FEL laser has been designed and is ready to be built with an output of 200 kW. It can be upgraded to one megawatt, and could supply the needed power for the thruster if a means to project that power to the satellite existed. The transfer mirror technology described here for developing approximately one meter diameter adaptive telescope optic mirrors will be a building block for these large adaptive optic laser beam projectors. Since their diameter is so large the beam intensity in the atmosphere is less than the intensity of sunlight and they will provide an eye safe approach to help to solve the power problem in space.
Background materials may be found in “Ground-based adaptive optic transfer mirrors for space applications: I and II” of the First International Symposium on Beamed Energy Propulsion, Huntsville, Ala., Nov. 5-7, 2002, to be published in book form by American Institute of Physics.